When a moving organ is photographed by an X-ray CT apparatus, artifacts due to motion, so-called motion artifacts are caused in an obtained cross-sectional image. As a typical method of reducing the motion artifacts, there is a method of controlling the acquisition timing of projection data or processing the acquired projection data on the basis of an electric signal acquired by measurement using biological sensors, such as an electrocardiograph or a respiratory sensor. Taking the photographing of a heart as an example, there is a method in which an electric signal acquired by measurement using an electrocardiograph, that is, electrocardiographic information is collected together with projection data and projection data corresponding to an arbitrary cardiac beat phase is extracted from the collected projection data on the basis of the acquired electrocardiographic information to perform image reconstruction and as a result, a cross-sectional image of the heart corresponding to the cardiac beat phase is obtained. This method is called an electrocardiographic synchronous reconstruction method.
In order to reduce motion artifacts in the electrocardiographic synchronous reconstruction method, it is important to designate as an optimal phase a cardiac beat phase corresponding to the projection data used for the image reconstruction, for example, to designate a cardiac beat phase with the smallest amount of movement of the heart. PTL 1 discloses an X-ray CT apparatus which calculates the amount of movement between cardiac beat phases by performing an addition process on the projection data and determines a low-motion cardiac beat phase on the basis of the amount of movement.